In electrical stations and sub-stations, there exists valuable electrical equipment mounted on supporting structures including steel columns that are attached to cement foundations. Such electrical equipment is of substantial value and its failure can disrupt electrical service within the service area of the electrical station or sub-station. Such disruptions can also have potentially damaging or disruptive ripple effects beyond the service area, on other parts of a power transmission system. Natural forces, such as earthquakes and high winds, and other external forces are a potential cause of failure for such equipment and for other types of valuable electrical and utility equipment, such as microwave signal towers, television and radio antennas, mobile telephone antennas, street lights, traffic signals, and the like.
The columns used in equipment support structures are commonly fabricated of steel pipes or beams (e.g., I-beams) welded to steel footings, which are bolted down securely to mounting studs embedded in concrete foundations. When forces such as a seismic event or high winds are applied, the column bends and the structure sways laterally. The severity, duration, and frequency of the forces; the column structural design; the column connecting system; and the mass of the equipment all impact whether the column and/or the equipment will sustain structural or functional damage from the forces. Thus, there is a need to isolate the electrical equipment and supporting columns from natural events, such as seismic events and high winds, and from other forces, in order to prevent damage to the equipment.
The column connecting system, where the base of the structure is typically securely bolted down to a concrete platform or other foundation, provides virtually no motion isolation for the electrical equipment, although a very small amount may be afforded by inherent flexibility of the column. To prevent damage and ensure the survival of the equipment and the structure, motion isolation systems are sometimes used to resist applied loads and to dampen vibration.
Single-acting spring isolators, which absorb energy in only one direction, are known for use in new equipment installations and for retrofitting existing installations. However, such isolators are installed between the concrete foundation and the base of the support structure in a dual-end-mount configuration. The present inventors have recognized that isolators of a dual-end-mount configuration are difficult to service, once installed. Furthermore, the inventors have noted that retrofitting in this configuration requires excessive installation expense, as the site must be shut down, the support structure must be detached from the foundation, and then the support structure must be reinstalled on the isolator. Before reinstallation of the support structure on the isolator, a section of the support structure equal in length to the isolator must typically be removed, which requires the base or other part of the structure to be rewelded and regalvanized.
U.S. Pat. No. 5,971,374 of Freeman describes a dual-acting seismic damper that dissipates energy in response to both upward and downward forces on large electrical equipment. The seismic damper described by Freeman is mounted between the equipment support structure and a foundation in a dual-end-mount configuration. Consequently, the damper of Freeman suffers from the same shortcomings in serviceability and retrofitting as described above. Additionally, it uses a single spring for resisting forces in both the upward and downward directions. The present inventors have recognized that the single spring configuration of Freeman does not provide effective damping during the rebound after upward or downward motion.
U.S. Pat. No. 3,856,242 of Cook describes a mounting apparatus for a surge voltage arrester comprising two sets of Belleville springs arranged on a bolt that extends from a mounting body through a hole in a supporting foot of the surge voltage arrester. One of the sets of Belleville springs is positioned beneath the foot of the surge voltage arrester, between a first side of the foot and the mounting body. The other set is positioned above the foot, between a second side of the foot and the bolt. As with the seismic damper of Freeman (U.S. Pat. No. 5,971,374), the mounting apparatus of Cook includes components mounted between the equipment support structure and its foundation. Consequently, it too suffers from the shortcomings in serviceability and retrofitting described above.
Thus, the inventors have recognized a need for an improved seismic isolator and for improved methods of retrofitting existing structures, such as columns supporting electrical or utility equipment.